1. Field of the Invention
The present invention relates to a superconducting electromagnet, and more specifically, to a magnetic shield structure of a motor for driving a cryocooler provided for the superconducting electromagnet.
2. Description of the Related Art
A superconducting electromagnet is used, for example, in a magnetic resonance imaging (MRI) machine. The MRI machine measures a nuclear magnetic resonance (hereinafter referred to as an NMR) signal detected from an electromagnetic wave which is obtained according to a spin state of a hydrogen nucleus with the NMR phenomenon; processes the signal; and creates, from the processed signal, an image of a hydrogen nucleus density distribution in a human subject's body. When the MRI machine measures the NMR signal, it is necessary to apply a strong static magnetic field (0.2 T or more) to an inside of an observation area thereof, in which a subject is placed, and to maintain the static magnetic field in a high uniformity (for example, a variation of about 10 ppm).
Strength of the NMR signal detected according to the spin state of the hydrogen nucleus is proportional to that of the static magnetic field. Thus, an image in a higher resolution requires an increased strength of the static magnetic field. In particular, to generate a high magnetic field (for example, 1 T or more) in an area for an image pickup, a superconducting electromagnet using a superconducting coil is necessary. The superconducting coil needs to be continuously kept at an ultralow temperature. The superconducting coil is thus housed in a coil vessel which is filled with a cooling medium such as liquid helium. The coil vessel is further housed in a vacuum vessel to prevent heat from conducting to the superconducting coil from outside (see U.S. Pat. No. 6,570,475).
Even with the vacuum heat insulation, however, heat of radiation is inevitably conducted to the superconducting coil. It is also difficult to avoid heat conduction thereto from components for supporting the superconducting electromagnet. Thus, a cryocooler is typically provided which has a cooling capacity surpassing heat quantity to be conducted to the superconducting electromagnet. The cryocooler cools the cooling medium in an ultralow temperature system of the coil vessel. One of such well-known cryocoolers is the Gifford-McMahon cryocooler, which is described in “H. Ogiwara, Introduction to Cryogenic Engineering, Tokyo Denki University, (1999)”. The Gifford-McMahon cryocooler is configured such that a displacer is inserted into a cylinder to define an expansion chamber, and a volume of the expansion chamber is changed according to a reciprocating movement of the displacer driven by a motor, to thereby generate an ultralow temperature with an adiabatic expansion. Then a wall of the cylinder surrounding the expansion chamber is used as a cold head, which is inserted into the coil vessel to cool the cooling medium.
In the meantime, if the motor of the cryocooler described above is subjected to a leakage magnetic field from the superconducting electromagnet housed in the coil vessel, the motor could stop, or a torque thereof could decrease, though depending on a strength of the leakage magnetic field. To prevent such problems, Japanese Published Patent Application, Publication No. HEI 6-188466 proposes a magnetic shield structure in which a motor is surrounded by flat plates made of a ferromagnetic material, and a hole is created in a position where the motor is connected to a displacer. Japanese Published Patent Application, Publication No. 2004-349477 proposes another magnetic shield structure in which a motor is partly covered with a ferromagnetic material to divert magnetic field lines which may otherwise permeate the motor.
“R. M. Bozorth, Ferromagnetism, D. van Nostrand, Princeton, N.J., p 849 (1951)” describes a relation between a shape of the ferromagnetic material and an easiness of permeating magnetic field lines therethrough (a demagnetizing field coefficient).
The magnetic shield structure according to Japanese Published Patent Application, Publication No. HEI 6-188466 is more desirable than that according to Japanese Published Patent Application, Publication No. 2004-349477. In the former, the entire motor is substantially covered with the ferromagnetic material, whereas in the latter, a leakage magnetic field may permeate the motor, part of which is not covered with the ferromagnetic material.
However, even the magnetic shield structure described in the former has the following problems:
1. The ferromagnetic material plates for covering the motor naturally needs an opening through which a power transmission shaft such as a motor drive shaft, a link mechanism, or the like passes. A leakage magnetic field may permeate the motor through the opening; and2. If the motor is almost entirely surrounded by the ferromagnetic material plates, maintenance of the motor is difficult to be performed.
In particular, a cryocooler having a driving mechanism generally has a shorter expected lifetime than the superconducting electromagnet. The cryocooler needs to be replaced by a new one sometime within the usage period. When such a cryocooler is first installed, pipes to connect the cryocooler to a compressor for compressing a cooling medium, a power cable to connect the cryocooler to the motor, and the like are also installed. Therefore, when the cryocooler is replaced by a new one later, a magnetic shield thereof and the motor must be also removed.
On the other hand, it is difficult to remove the magnetic shield, because an electromagnetic force of, for example, as strong as about one ton is applied to the ferromagnetic material covering the motor, while the superconducting electromagnet is in operation. It is preferable in general that the cryocooler is continuously operated without demagnetizing the superconducting electromagnet. In the conventional cryocooler, however, when the magnetic shield is removed, the superconducting electromagnet has to be demagnetized. This increases the time necessary for maintenance operations of the cryocooler.
The present invention provides a magnetic shield structure allowing maintenance of a cryocooler to be performed without demagnetizing a superconducting electromagnet, and protecting a motor for driving the cryocooler from a leakage magnetic flux of the superconducting electromagnet.
The inventors have found the following, based on the technology described in “R. M. Bozorth, Ferromagnetism, D. van Nostrand, Princeton, N.J., p 849 (1951)”. The inventors herein assume that a pair of plate-like ferromagnetic materials are provided, both of which extend in a direction of the magnetic field lines of the superconducting electromagnet. If a length of each ferromagnetic material plate in the extending direction (in the direction of the magnetic field lines) is sufficiently longer than a thickness thereof, each ferromagnetic material has a smaller demagnetizing field coefficient, and permeates more magnetic field lines therethrough. That is, when the ferromagnetic material plates are provided in a magnetic field generated in a certain space, the magnetic field lines which are present around the space are also attracted to the ferromagnetic material plates, and are made to permeate therethrough. As a result, the magnetic field around the ferromagnetic materials is reduced.